The present invention relates to the correction of errors occuring in the transmission of data and more particularly to the correction of errors occurring in the transmission of differentially encoded quadrature phase shift keyed data (DQPSK)
Differentially encoded quadrature shift keying (DQPSK) is a highly efficient modulation technique for satellite communication channels. In a typical DQPSK system each sequential two binary bits of information in a string of binary bits i.sub.1, i.sub.2. . . i.sub.n causes a shift .theta. in the phase .psi. of the carrier signal by the mapping of the combination [(0,0), (0,1), (1,1), (1,0)] of the two bits i.sub.n, i.sub.n.sub.+1 into shifts, .theta., of [0.degree., 90.degree., 180.degree., 270.degree.] respectively in the phase .psi. of the carrier signal. To put it another way, with DQPSK, information .theta. on each two sequential bits i.sub.n, i.sub.n.sub.+1 is encoded as the difference between successive phases, .psi..sub.i.sub.+1 - .psi..sub.i.
The advantages of phase shift keying (PSK) is that it is more efficient than frequency shift keying and the advantages of differential phase shift keying (DQPSK) is that it eliminates the need to transmit a reference phase to prevent ambiguity in decoding of the transmitted data. However, one disadvantage of DQPSK is that if a single bit error or an error of 90.degree. in the phase occurs during the transmission of .psi..sub.i, the output of the differential decoder will contain two single-bit errors: one in the estimate of .psi..sub.i - .psi..sub.i.sub.-1 and one in the estimate of .psi..sub.i.sub.+1 - .psi..sub.i. Thus, the bit error rate is doubled and bit errors are correlated. The correlation of errors is the more serious problem because it severely degrades the efficiency of a random-error correcting code. If, for example, a single-error correcting convolutional code is used for forward error correction, there is no guarantee that it will correct any double-bit errors.
One solution to the problem is to perform error correction before differential decoding of the data. In this scheme, the error correction code (ECC) decoder need not contend with double-bit errors, but is faced with the same phase ambiguity that the differential encoding is used to resolve. Techniques to resolve this ambiguity include an acquisition search at start-up and whenever the modem undergoes a 90.degree. phase slippage; much of the benefit of differential encoding is lost.
Another approach is to transmit adjacent bit pairs on different channels and then interleave the outputs so that single-bit errors are not automatically mapped into double-bit errors. The difficulty with this approach is that it increases the amount of hardware necessary to transmit and correct the data and increases the error rate in the transmitted data.
The other alternative approaches would be to use a code that corrected errors occuring in 4 bit bursts or to use a two bit error correction code to correct for single-bit phase errors. Both of these techniques are very inefficient.